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My elderly neighbor has a 1998 Expedition with an alternator problem as all of the units I've installed and tested output less than 12v.

He wants to save some money and purchases used alternators from junkyards and winds up returning them. So far, I've installed 3 alternators and they each show different voltage readings with no loads: 11.75v, 11.82v, 11.88v. With a load (lights, A/C, etc.) the voltages drop into the 11.2v to 11.3v range, regardless of how much I rev the engine.

With the car off, the battery shows 12v and when the car is running, it will be less than 12v.

Are all the alternators bad or is there something else causing the low voltage problem? The power cable from the alternator to the battery is fine as is the negative (-) cable to the chassis.

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  • He is saving money by having a nice person like you continually changing cheap alternators. He has wasted time and money at the junkyard, have him buy one at the parts store and be done with it. Low battery charge can cause the output reading of the alternator to indicate low voltage, use a battery charger to full charge the battery before reading voltage at alternator.
    – Moab
    Mar 26, 2020 at 22:07
  • A fully charged battery should be showing about 14V with the engine running but no A/C, lights, etc. When you switch the engine off it should drop quickly (a few minutes) to about 13V and then more slowly (several hours) to around 12.6V. Try swapping the battery from another car. The alternators may all be working correctly.
    – alephzero
    Mar 27, 2020 at 0:33

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Are you checking the voltage at the alternator output terminal? if not you could have a bad connection causing voltage drop. First test of voltage right at the alternator output(Make sure to ground to the alternator as well) if you measure 14 volts at the alternator you will have to test for voltage drop on the positive and negative. To test for voltage drop turn the car on and turn on some electrical loads (defroster, radio, etc), then put your multimeter (in voltage mode) across the alternator positive and battery positive, More than 0.6 volt drop is cause for concern, do the same for the negative. It could also be that some high electrical load device or short is coming on with the vehicle, you can check this with an amp clamp around the battery cable of course it could also just be a case of a bunch of bad alternators.

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The alternator should show around 13-14 volts at startup. This alternator has an internal voltage regulator so it should be good to go as soon as it's installed. Check the terminals inside the connector and check the wiring harness for opens to rule out a wiring issue.

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Check the bridge diode circuit inside the alternator/generator. Sometimes, there is also a wiring harness/loom which is faulty for this kind of situations.

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